I think I'm toward the "power user" end in terms of email usage, and Thunderbird works wonderfully for me. I briefly tried to use Apple's Mail.app many moons ago, but its handling of shared IMAP folder subscriptions, caching, etc. was atrocious. Outlook for the Mac is even worse.

Thunderbird isn't the prettiest UI in the world, but it handles my three email accounts (one Exchange, two IMAP) flawlessly. It does X.509 and GPG encryption/signing with very little fuss, and with the help of an extension, it even shows me a nice visualization of email threads that I can click on to navigate threads. Searches by subject, sender, body text are all quite fast. It handles multipart/mime messages properly, it has many options for filtering out HTML email cruft and just showing me the text in a proper monospace font... What else could anyone want from an email program?